On the Old Red Sandstone of the North of Ireland

Abstract

I n the North of Ireland there are two distinct kinds of rock, classed both on Griffith's and Portlock's Geological Maps as Old Red Sandstone. The lower and larger member of this group occupies a considerable area, having an extent of about thirty miles in length by an average width of ten miles, from Lough Erne north-eastwards to Pomeroy in Tyrone. It consists for the most part of dark red and purple conglomerates, often coarse and massive, and of purple, and sometimes, though rarely, greenish-grey, pebbly and fine-grained sandstones, often micaceous and in some cases calcareous, with sandy shales. The pebbles in the conglomerate, which vary from the smallest size up to blocks over a foot in diameter, consist of purple felstone, grits, schist, and quartzite. Of all these the felstone pebbles are by far in greatest proportion, the rock being in some places almost entirely composed of them; and their source is unquestionably certain tracts of igneous rock which will be presently described. On the north and north-west these conglomerates, sandstones, &c.are bounded by metamorphic rocks, from which they are separated by a fault ; but in the north-east, near Pomeroy, they rest unconformably upon fossiliferous slates and grits of Lower Silurian age. Though the discordance between these formations is not actually seen in section, yet there can be no doubt of its existence, the southern extension of the Lower Silurian rocks occupying a semicircular area, in which the strike is east and west, while the red sandstones that margin